Conventional rate/distortion (RD) optimization in video encoders assumes that optimization decisions made for a current frame are independent of optimizations in future frames. In practice, the assumption is not correct. It is sometimes better to choose a locally sub-optimal setting for a macroblock in one frame in order to improve the quality of future frames. Solving the co-optimization problem exactly is extremely expensive.
It would be desirable to have a mechanism for inexpensively approximating the correct co-optimization solution.